Substantial experimental and epidemiological data indicate that 1,25-dihdroxyvitamin D3 (calcitriol) has potent antiproliferative effects on human prostate cancer cells. We performed an open label, nonrandomized pilot trial to determine whether calcitriol therapy is safe and efficacious for early recurrent prostate cancer. Our hypothesis was that calcitriol therapy slows the rate of rise of prostate specific antigen(PSA) compared with the pretreatment rate. After primary treatment with radiation or surgery recurrence was indicated by rising serum PSA levels documented on at least 3 occasions. Seven subjects completed 6 to 15 months of calcitriol therapy, starting with 0.5 ug. calcitriol daily and slowly increasing to a maximum dose of 2.5 ug. daily depending on individual calciuric and calcemic responses. Each subject served as his own control, comparing the rate of PSA rise before or after calcitriol treatment.